Testsuite [WebDAV]: Use keep-alive.
Fixes multiple issues on Solaris with the WebDAV testsuite.
* Support multiple Protocol objects for the same URL
(on Solaris there is one Protocol object each for IPv4 and IPv4).
* Fixed some failures "I/O error: Address already in use", by
using HTTP keep-alive for the WebDAV requests.
These failures were in part due to Solaris holding closed TCP
connections in TIME_WAIT for 4 minutes (Linux 2 minutes).
* Also a workaround for Protocols.HTTP.Query not handling the
HEAD method in keep-alive replies.